Fatigue crack growth and fracture toughness behavior of an Al-Li-Cu alloy
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I.
INTRODUCTION
AIRCRAFTdesigners
are constantly striving to achieve minimum weight in order to cut fuel consumption and improve overall performance. Reducing the density of structural materials has been shown to be the most efficient solution to this problem.~ Since aluminum alloys make up between seventy and eighty percent of the current aircraft weight, recent alloy development programs have focused on reducing the density of these materials. 2 Lithium additions to aluminum provide the greatest reduction in density of any alloying element and offer the additional advantage of increasing the elastic modulus. However, AI-Li-X alloys often exhibit low ductility and fracture toughness. Various modifications in alloy chemistry and fabricating techniques have been used in an attempt to improve the ductility while maintaining a high strength. Copper, Mg, and Zr solute additions have been shown to have beneficial effects.3 Magnesium and Cu improve the strength of AI-Li alloys through solid solution and precipitate strengthening, and can minimize the formation of precipitate free zones (PFZ) near grain boundaries. Zirconium, which forms the cubic AI3Zr coherent dispersoid, stabilizes the subgrain structure and suppresses recrystallization. The goals of most AI-Li-X alloy development programs include improvements in density and modulus with equivalent or improved damage tolerance and corrosion properties compared with currently used materials, e . g . , 7075 and 2024. 4 Although there have been numerous reports on the relationship among composition, microstructure, and monotonic properties of AI-Li-X alloys, 5'6 there have been few studies on the cyclic properties and fracture toughness of these materials. This paper describes the fatigue crack propagation and fracture toughness of a new alloy based on the AI-Li-Cu system which is somewhat related to the AI-Li-Cu alloy 2020 that was commercially available in the 1960's. II.
EXPERIMENTAL PROCEDURES
The actual chemical composition of the AI-Li-Cu alloy used for the present investigations is shown in Table I. It is K.V. JATA, Research Assistant Professor, Department of Materials Science, and E. A. STARKE, Jr., Earnest Oglesby Professor of Materials Science and Deaa, School of Engineering and Applied Science, are with The University of Virginia, Thornton Hall, Charlottesville, VA 22901. Manuscript submitted April I1, 1985. METALLURGICAL TRANSACTIONS A
similar in composition to the AI-Li-Cu alloy, M2, recently studied by Feng et al. ,7 except for slightly lower Cu and Cd and slightly higher Li contents. In addition to the above, low levels of Fe and Si were maintained to minimize the amount of constituent phases. Zirconium was added as the dispersoid forming element. The material was obtained from Reynolds Metals Company in the form of 27.7 mm thick plates. The original cast ingots were homogenized in an argon atmosphere as follows: (i) heated at 523 K/hour to 673 K, held 48 hours, (ii) heated at 298 K / h o u r to 763 K, held 18 hours, (iii) heated at 298 K/hour to 788
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